The invention relates to a power converter comprising a DC input connected to an inductive element, a switch arranged to interrupt the inductive current cyclically with a predetermined cycle period, a plurality of load circuits, and a multiplexer having a plurality of parallel channels between first and second output terminals of the inductive element, wherein each channel includes one of the load circuits, and the multiplexer is adapted to make the channels conductive sequentially and periodically.
More particularly, the invention relates to a power converter that can be used as an LED driver, especially for high luminance LEDs for illumination purposes.
LED illumination devices, e.g. devices that may be connected to the power grid by being screwed into a socket like a conventional light bulb, may be equipped with white LEDs or with combinations of colored LEDs, e.g. in the basic colors red, green and blue (RGB). Combinations of colored LEDs are superior to white LEDs in terms of life time and have the additional advantage that, by individually controlling the intensity of the different colors, the color temperature of the light source may be adapted as desired, and drifts in color temperature that may be due to aging or different temperature characteristics of the LEDs can easily be compensated. However, a drawback of colored LEDs is that separate current-controlled power supplies must be provided for the different colors.
It is possible of course to provide an individual power converter for each color, i.e. for each group of LEDs that have the same color and may be connected in series or in parallel. This solution, however, is relatively expensive, because at least three power converters are needed for the at least three colors that are necessary for obtaining white light.
Another approach is to provide a transformer with a plurality of secondary windings, so that each load circuit (group of LEDs) may be connected to a different one of the secondary windings. In this case, however, the extra windings lead to an unfavorable transformer (size, cost, efficiency), and the necessity to provide post-regulators further reduces the efficiency and increases the costs. Yet another alternative is to connect the load circuits in series in the secondary circuit of the transformer. In this case, however, The LEDs must be selected in accordance with a very strict classification (binning) in terms of efficiency, color, and/or forward voltage, or additional circuitry is required in each load circuit for individually controlling each of the output powers, which adds to the complexity and costs of the converter as a whole and makes it more difficult to galvanically isolate the output circuits from the potentially harmful grid voltage.
US 2006-072252 A1 and WO 03/041254 disclose power converters of the type indicated in the opening paragraph, wherein the multiplexer subdivides each energy packet that is delivered by the switched inductive element into a number of fractions to be directed to different outputs.